Where can I find the custom bone in version 2.8?
It looks like the Collection 11, the one with your bones, was hidden, I don’t know why as I don't know a lot yet about how the switch from 2.79 layers to 2.8 collections works, but anyway unhide it:



About custom bones, how to create custom bones:


In Object mode, create the object that is supposed to be used as the bone's custom shape, for example a sphere.
Select your armature, switch to Pose mode, select the bone you want to custom.
In the Properties panel > Bone > Viewport Display, enable Wireframe so that your custom bone will be displayed as a wireframe, and choose your sphere as Custom Object.
Now your bone is displayed as a sphere in Pose and Object mode.
You can modify the original sphere object (edit, rotate, scale...), it will be directly replicated on the custom bone. But you can scale in the same Viewport Display menu.